Artist rendering of the Chinatown subway station indicating stair/escalator and elevator access. (SFMTA)

SAN FRANCISCO (KCBS/BCN)—A tunnel-boring machine that has been slowly carving out the path for San Francisco’s new Central Subway system will pass under Market Street next week and could cause BART delays, Municipal Transportation Agency officials said.

“We’re making better progress than anticipated, having hit as much as 100 feet a day—significantly more than we had planned or scheduled,” SFMTA Director of Transportation Ed Reiskin said in a statement.

The tunnels will be located about 10 feet below the BART tunnel, and will eventually form a new branch off the T-Third line that will connect the city’s South of Market neighborhood to Chinatown, with stops at Yerba Buena/Moscone and Union Square/Market Street, according to the SFMTA.

Mom Chung will be in operation 24 hours a day for the work below Market Street, and will then head north to Stockton Street.

The crossing under the BART tracks is expected to take up to six days, and could cause BART trains to run at slower speeds through the area. Information about BART delays will be available at www.bart.gov/alerts or by calling 511.

“Working closely with BART and some of the top tunneling experts in the country, including our tunneling contractor, we will build this new tunnel under BART safely and soundly,” Reiskin said.

About 150 monitoring devices have been placed in the Powell Street Station and neighboring buildings to allow SFMTA officials to receive live data feeds about tunneling progress.

“We feel really good about it. We are using the right technology; we’re using state of the art TBMs (tunnel-boring machines) and we’re constantly monitoring the ground conditions around the tunnel and on the surface,” Sarah Wilson, the resident engineer overseeing the tunnel boring, said.

More than 2,100 feet of tunnel has been created already underneath Fourth Street.

Meanwhile, Big Alma, another tunnel-boring machine, is expected to cross under Market Street in January to create a second tunnel parallel to the first one for future trains running in the opposite direction, SFMTA officials said.

The $1.6 billion Central Subway is expected to open in 2019. Updates on the project can be found online at www.centralsubwaysf.com.

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